docs: add external contribution quick guide

[ryan-lempka]

Summary

Adds an External Contributions section at the top of CONTRIBUTING.md so first-time contributors see the end-to-end flow before the internal development details. Modeled on the ai-dynamo/dynamo contribution guide, trimmed to the essentials:

• Fork-and-clone workflow (the existing setup instructions assume push access to this repo, which external contributors do not have)
• Size-based process: small fixes go straight to a PR, larger changes (~100+ lines or new features) start with an issue
• DCO sign-off step, pointing at the existing Signing Your Work section
• Note that reviewers are requested automatically via CODEOWNERS
• Expectation to understand and be able to explain AI-assisted code

Also includes a few small related fixes:

• Internal workflow now notes the DCO sign-off requirement (the DCO check is required on every PR as of today), and the stale note about which status checks should be required now reflects reality (CI Success + DCO)
• Issue-template contact link for questions pointed at the wrong org (NVIDIA/switchyard, a 404); now points at this repo's Discussions, which is enabled
• detect-secrets baseline refreshed (line numbers shifted by the CONTRIBUTING.md insertion)

[coderabbitai]

Walkthrough

CONTRIBUTING.md is updated with a new "External Contributions" section describing the end-to-end workflow for external contributors, including forking, workflow selection, branching, DCO sign-off, PR submission, CODEOWNERS review, and expectations around AI-assisted code changes.

Changes

Contributing Guide Update

Layer / File(s)	Summary
External Contributions section CONTRIBUTING.md	Adds documentation covering fork/clone setup, workflow selection by change size, branch and required checks, DCO sign-off, PR submission against main, CODEOWNERS-driven review, keeping PRs focused with tests, and AI tool usage expectations.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

A rabbit hops with docs in paw,
New guidance added, clear and raw,
Fork it, branch it, sign your name,
DCO stamped upon the frame,
Hop along, contributors all! 🐇📜

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main docs change: adding guidance for external contributors.

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_{Comment @coderabbitai help to get the list of available commands.}

[ayushag-nv]

@ryan-lempka This CI secret scan continuously fail, can we have long term fix in this PR or a follow up ?

[ayushag-nv]

Looks Good. Approving